1. Field of the Invention
This invention relates to a developer supply device for maintaining constant the developing effect of a developer for electrophotography having a toner and carrier.
2. Description of the Prior Art
Generally, in the developing method in dry type electrophotography or electrostatic recording, use is made of a two-component developer consisting of a mixture of toner and carrier. It is well known that the weight admixture ratio of toner and carrier in such a developer is a very important factor from the viewpoint of the developing effect. When the proportion of toner to carrier (hereinafter simply referred to as the toner density) is low, namely, when the density of the developer is low, the density of the developed image becomes thin. Conversely, when the proportion of toner is too high, the density of the developed image becomes too thick and also fog is increased.
Accordingly, in order that images of preferable color tone may be continuously obtained, it is necessary to render the density of the developer into a proper level and moreover to maintain the level constant always during development. To control the toner density constant, there are:
(1) a method in which a toner and carrier are made to differ in color and by utilizing the fact that when the toner density varies with the consumption of toner, the color of the developer which is a mixture of toner and carrier changes, the color change is optically detected so that a toner supply mechanism is controlled in accordance with that change, thereby maintaining the toner density constant; PA1 (2) a method in which a portion to be experimentally developed (for example, to be developed by applying a voltage to a conductor of a color different from the color of the toner) is provided and the reflection density of the developed portion is optically detected so that a toner supply mechanism is controlled in accordance with the variation in the reflection density to thereby maintain the toner density constant (see, for example, U.S. Pat. No. 3,348,521); PA1 (3) a method in which a transparent conductor such as a Nesa glass electrode or the like is used to effect experimental development as in the method (2) above and the transmission density of the developed portion is optically detected so that a toner supply mechanism is controlled in accordance with the variation in the transmission density to thereby maintain the toner density constant; and PA1 (4) a method in which the difference in magnetic permeability between the toner and carrier is utilized to detect any variation in magnetic permeability of the developer so that a toner supply mechanism is controlled in accordance with the variation to thereby maintain the toner density constant (see, for example, U.S. Pat. No. 3,698,926).
However, in the conventional developer supply devices, although the developer handled thereby has been a two-component developer, the developer supplied as a replenishing agent has been toner alone or a developer consisting of toner and carrier mixed therewith at a very small proportion. The conventional developer supply devices have not been such that the supply of carrier is effected separately from the supply of toner. Accordingly, there have been adopted means whereby toner alone or toner with a small amount of carrier mixed therewith is supplied to compensate for the reduction in toner density which takes place in accordance with toner consumption, and this has led to a disadvantage that even if the toner density can be maintained substantially constant, the amount (volume) of the developer (mixture of toner and carrier) within the developing device varies. Also, in the conventional developer supply devices, the supply of carrier is not independently effected and therefore, when toner alone is supplied, a variation in volume of the developer has resulted from the consumption of carrier (actually, carrier is also consumed little by little), and in the case of the supply of toner with carrier mixed therewith, there has been a disadvantage that if a balance with the consumed carrier is not kept, a variation in volume of the developer may also occur to vary the image density.
Such a variation in volume of the developer within the developing device cannot be visually confirmed because the developing device is usually mounted deep in the dark interior of an electrophotographic apparatus body and heretofore, it has been difficult to judge such variation by referring to the density of the image on the resultant copy paper or to detect it only when the developer has become less than a predetermined amount. Accordingly, the fact that it cannot be easily determined if there is any change to increase or decrease the amount of the developer from a preferable amount has necessarily resulted in adverse effects such instability of the image density attributable to the variation in amount of the developer, increased fog, reduced sharpness of the image portion, irregularity of development etc., and has also adversely affected the stirring and conveyance of the developer.
Now, the variation in amount of the developer within the developing device will be positively explained. FIGS. 1 and 2 of the accompanying drawings show consumption data, actually measured of toner and carrier by the magnet developing method using magnetic carrier (iron powder) and toner as a two-component developer.
FIG. 1 shows the toner consumption by a printed image, namely, a printed image in which the proportion of the area occupied by the black print of the toner to the white ground of recording paper differs. The ordinate represents the weight of toner consumed per sheet of A4 size recording paper. The abscissa represents the proportion of the area of the print portion (the portion to which toner is to adhere) to the area of the recording paper (in this case, A4=210 mm.times.297 mm). The term 100% used herein means a solid black image.
FIG. 2 shows the carrier consumption resulting from the difference between printed images. The ordinate represents the weight of carrier consumed per sheet of A4 size recording paper. That is, the ordinate shows the measured amount of carrier on a developed member (in the present measurement example, a photosensitive drum). The abscissa, as in FIG. 1, represents the proportion of the area of the print portion to the area of the recording paper.
Part of the toner or carrier of the amount shown in FIGS. 1 and 2 is transferred to the recording paper and the remainder is collected by a cleaning device. That is, these amounts of toner and carrier show the amount brought out of the developing device.
As can be seen from FIGS. 1 and 2, the toner consumption varies in accordance with the area of the print portion of the printed image. On the other hand, the carrier consumption is not varied as much. This shows that the rates of consumption of toner and carrier are varied by the printed image. In spite of the fact that the proportion of toner and carrier consumed thus differs depending on the printed image, an attempt to maintain the toner density constant by one kind of replenishing agent (toner alone or a replenishing developer consisting of toner with carrier mixed therewith at a certain predetermined ratio) as has heretofore been done would necessarily result in a variation in amount of the developer within the developing device.
A method of detecting any variation in volume of the developer within the developing device and supplying toner to maintain the volume of the developer constant is heretofore known from Japanese Laid-open Patent Applications Nos. 19459/1975 and 78343/1976. This method is effective to maintain the density of the toner in the developer constant for a relatively short time (the order of 100,000 to 200,000 sheets of A4 size recording paper). However, this method is based on the premise that the amount of carrier brought out of the developing device is zero or constant and therefore, in a case where the developing device is used for such a long time as to exceed 500,000 sheets of A4 size recording paper, the density of the toner in the developer within the developing device comes to greatly differ from its initial value and this will remarkably affect the output images. Particularly, in an electrophotographic apparatus utilizing a laser beam, image formation is effected continuously and for example, an average of 1,000,000 sheets of A4 size recording paper is produced per month. This will result in a remarkable decrease not only in toner but also in carrier.